The present invention relates to a protection apparatus for use with a spot network substation or a main/sub 2-line receiving substation, and more particularly to a protection apparatus for a substation having a circuit capable of generating a reverse power flow to a power source at an electric power company.
A spot network substation is well known in which reverse power flow is generated. Since power supply reliability is very high, the spot network substation is widely used for building substations. As disclosed in Japanese Patent Application Laid-open No. JP-A-7-68624, in a conventional protection system for a spot network substation, a plurality of receiving lines are connected between three-phase and three-line power sources and a network bus connected to circuit breakers (CB's) for supplying power to an air conditioner, an elevator, and the like. Each of the plurality of receiving lines is connected to a primary switching device, a network transformer, a protector CB and a current transformer in succession. For example, the current transformer detects reverse energizing current upon energization of the network transformer by the network bus when a power source is stopped. This detected reverse energizing current causes a network relay connected to the current transformer to judge a presence of a reverse power flow state, i.e., a power-stop of the power source and trip the protector CB to protect the spot network substation. This system is collectively called a spot network protector.
In this spot network substation, in order to deal with an emergency when power supply from power sources is stopped because of some accident, an emergency generator is connected to one end of the network bus to supply power to an emergency facility only when power supply to all the receiving facilities is stopped. Recently, demands for efficient use of facilities have become intense and switching to a co-generation system as an uninterrupted generator has been desired.
Various types of power sources such as spot network power sources and main/sub 2-line power sources have been used recently depending upon power user requirements. These various types of power sources are required to be managed collectively because of demands for efficient use of power sources. Furthermore, in order to improve reliability of power supply of spot network power sources over a conventional system, for example, some three-line spot network power sources supply power from different transformers and different substations. Collective management of these power sources with different transformers generates a voltage difference between power sources.
However, the spot network protector for a spot network substation has a reverse power tripping function as described above. If a generator connected to a power line is operated, current flowing from the network bus connected to the generator through the power source causes to trip the protector CB. Therefore, this system is difficult to use if the generator is to be operated at high efficiency such as in the case of a co-generation system. Furthermore, an emergency generator cannot be switched unless a full power-off is effected which is therefore inevitable.
If the voltages of power sources become unstable, voltages between a plurality of receiving lines of the spot network substation become different. Therefore, power is supplied from a high voltage receiving line to a low voltage receiving line, and a reverse power tripping function operates at the low voltage receiving line so that the spot network substation cannot operate normally.
If another substation such as a main/sub 2-line receiving substation is connected to spot network power sources and a generator connected to a power line is used, there is no reverse power tripping function. In this case, therefore, power is transmitted to a stopped spot network power source. As described above, currently used protection units cannot collectively manage spot network power sources, main/sub 2-line receiving power sources, and the like, in terms of operation safety and reliability.